Shoot Em Up: Create enemy movement

[dumbanode]

Instead of having the enemies stay completely still. Add some variation to their movement.

For example, enemies can move in:

• Space Invaders style
• Galaga style
• Xevious style

[Zshandi]

Perhaps this should be broken into 4 parts: 1 for each of the styles, and one for the ability to randomly pick a style for each instance.